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Spectroscopic characterization of MoS2 monolayers grown on a Graphene/Iridium substrate

In the wake of the discovery of graphene, research on two-dimensional (2D) materials has been a very successful field over the last 15 years. One of the most popular 2D materials is molybdenum disulphide (MoS2). This material is a direct bandgap semiconductor and thus potentially important for use in 2D electronics and photonics. Most research over the last years was done on exfoliated flakes from bulk crystals. While this method provides high quality samples for scientific measurements, the approach is not scalable and suffers from poor reproducibility. A promising alternative over the past year was the development of direct growth methods of monolayer MoS2 on metallic substrates using molecular beam epitaxy (MBE). These growth methods provide >1 cm2 sized samples opening the material up to scientific methods requiring bigger sample sizes as well as providing a way to applications. However, MoS2 monolayers grown on metallic substrates suffer from the strong interaction of MoS2 with the substrate. This limits the applicability in optoelectronics because of luminescence quenching. It is therefore of great interest to understand the properties of MoS2 grown on a van der Waals substrate such as graphene.
Combining angle-resolved photoemission spectroscopy (ARPES) taken at the BaDElPh beamline in Elettra with x-ray photoemission spectroscopy (XPS), Raman spectroscopy and photoluminescence (PL), N. Ehlen and colleagues from Germany and Italy investigated the electronic and optical properties of MBE grown MoS2 on a graphene/Ir(111) substrate. Using ARPES they were able to investigate the interplay of MoS2 and graphene and found that while graphene is donating electrons to the molybdenum disulphide layer, the interaction between both layers is limited. This result was confirmed via temperature dependent Raman measurements taken in an UHV Raman setup thus preventing the effect of molecule adsorption or intercalation. The Raman experiments are a sensitive probe of the bond length and showed that the thermal expansion of MoS2 is not following the expansion of graphene but rather expands and shrinks similar to what would be expected for a freestanding layer.
Photoluminescence spectroscopy showed a peak located at about 1.945 eV with a full width at half maximum (FWHM) of only 18 meV. The existence of such a sharp peak is surprising as the metallic substrate would be expected to quench any photoluminescence. Using the insight provided by the other methods, the authors conjectured that the appearance of PL can be explained by the weak coupling of the monolayer to the substrate.
To extract an exciton binding energy from the luminescence measurements, ARPES spectra of the cesium doped MoS2/graphene/iridium system were combined with theoretical calculations on the bandgap renormalization. In conjunction with position of the PL peak it was possible to extract an excitonic binding energy of about 480 meV.
The experiments show the synergies that can be created by combining different spectroscopic methods such as PL, Raman and ARPES for the characterization of new materials.

Figure 1. (a) ARPES spectra of MoS2 on graphene/Ir(111). (b) A cut through the ARPES spectrum at the point indicated via KMoS2 in panel (a) shows an energy splitting of the MoS2 bands of 144 meV (also visible by eye in the inset in (a)) due to spin-orbit coupling. (c) Fermi surface of the graphene Dirac states showing a small p-doping with respect to graphene/Ir(111) system. (d) After donation of electrons to the material via cesium doping a new band appears at the Fermi level (a closeup is shown in (e)) making it possible to extract a bandgap for the Cs-doped MoS2/graphene/iridium structure.